What type of subordinate clause is in the following sentence: "The trouble was that he forgot where he put his keys."?
There are two subordinate clauses in the sentence:
"that he forgot"
"where he put his keys"
A subordinate clause is a group of words that has a subject (he) and a verb (forgot & put) but is not a complete thought; it is dependent on (subordinate to) the main sentence (The trouble is).
The clause "that he forgot" is a relative clause, introduced by the relative pronoun "that". The antecedent of the relative clause is "trouble".
The clause "where he put his keys" is an adverbial clause, introduced by the adverb "where". The adverbial clause modifies the verb "forgot".
The compound subordinate clause "that he forgot where he put his keys" is functioning as a subject complement following the linking verb "is".
A linking verb is a verb that acts as an equals sign, the object of the verb is or becomes the subject. The compound subordinate clause = the subject of the sentence (trouble).